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PHIVOLCS Volcano Monitoring PDF Print
Friday, 09 May 2008 12:00


Why does the Philippines have many volcanoes?

The Philippines sits on a unique tectonic setting ideal to volcano formation. The archipelago is surrounded by subducting plates as manifested by the trenches that are related to volcano formation.


Classification of Volcanoes


1) Active volcanoes: erupted within historical times (within the last 600 years), accounts of these eruptions were documented by man; erupted within the last 10,000 years based on analyses of datable materials.


2) Potentially active volcanoes: morphologically young-looking but with no historical records of eruption.


3) Inactive volcanoes: no record of eruptions; physical form is being changed by agents of weathering and erosion via formation of deep and long gullies.


Volcanic phenomena directly associated with eruption

Lava flow, dome growth

Pyroclastic flow, pyroclastic surge, lateral blast

Tephra fall - ash fall, volcanic bomb

Volcanic gas


Volcanic phenomena indirectly associated with eruption

• Lahar, flooding
• Debris avalanche, landslide
• Tsunami, seiche
• Subsidence, fissuring
• Secondary/hydrothermal explosion
• Secondary pyroclastic flow.

Types of hazards posed by an active volcano


Lava flow is a higly elongated mass of molten rock materials cascading downslope from an erupting vent. The lava flow being extruded has low silica and low water contents.

Rate of flow: 3 km/day (slightly high viscosity) or 45 km/hour (low viscosity). Speed and geometry of lava flows depend on local topography. Steep slopes encourage faster and longer flows than gentle slopes or terrain


Lava dome is a pile or mound of lava that grew on the floor of an active crater, on the side slopes via a feeder vent that breached through the surface of the edifice, or inside the volcanic edifice.

Types: Exodomes - lava domes that were formed on the surface of the volcanic edifice)

Cryptodomes - lava domes that grew anywhere inside the edifice


Pyriclastic flow refers to hot dry masses of fragmented volcanic materials that move along the slope and in contact with ground surface. This includes: pumice flow, ash flow, block-and-ash flow, nuee ardente and glowing avalanche.


Pyroclastic flow mechanism:

Nuee ardente is a glowing eruption cloud characterized by: extreme heat (about 500 °C or higher)

1. high gas contentrapid flow down the slope of an erupting volcano enormous amounts of ash and other fragmental volcanic materials

A nuee ardente may originate directly from an active crater or from a collapse of a growing lava dome.


Pyroclastic surges are turbulent low-concentration density currents of gases, rock debris and in some cases, water, that move above the ground surface at high velocities.

Types: Ground surge, Ash-cloud surge, Base surge


* Hot blasts arise when pent-up gases facilitate their way out through the impermeable overlying materials and cause a very rapid escape into the atmosphere. Blasts that are directed obliquely often do much damage and could exact a high toll in human lives.
* Lateral blasts are combination of pyroclastic flows and pyroclastic surges with an especially strong initial laterally-directed thrust. They have an initial velocity of 600 kph and slow down to about 100 kph near its margin 25 km from the volcano.


Tephra falls may consist of pumice, scoria, dense lithic materials or crystals or combination of the four.

Particle size: less than 2 mm diameter (ash) , 2-64 mm diameter (lapilli) , more than 64 mm diameter (blocks and bombs)


Volcanic gas is one of the basic components of a magma or lava. Active and inactive volcanoes may release to the atmosphere gases in the form of: water vapor, hydrogen sulfide, sulfur dioxide , carbon monoxide, hydrogen chloride and hydrogen fluorid.

Aside from the major constituents, minor amounts of nitrogen, methane, argon and helium may be also present in volcanic gases. The proportion of these components changes with changing temperature.


Lahar (an Indonesian term), sometimes called mudflows or volcanic debris flows, are flowing mixtures of volcanic debris and water. Lahars are classfied into: Primary or hot lahar - associated directly with volcanic eruption and Secondary or cold lahar - caused by heavy rainfall.

Lahar distribute and redistribute volcanic ash and debris deposited around the volcano after the materials has cooled and has become water logged.

Lahar in tropical areas can be produced by:

* sudden draining of a crater lake, caused by either an explosive eruption or collapse of a crater fall (e.g. Agua, Kelut, Ruapehu)
* movement of a pyroclastic flow into a river or lake, displacing and mixing with water
* avalanche of water-sustained rock debris, where water can be from heavy rain, hydrothermal activity or other sources
* torrential rainfall on unconsolidated deposits on slope of a volcano (e.g. Pinatubo)
* collapse of a temporary dam, where recent volcanic deposits have blocked a steam channel (e.g. Asama, Pinatubo


Tsunami are long-period sea waves or wave trains that are generated by the under-the-sea earthquake. Most tsunamis are caused by fault displacements on the sea floor and of volcanic sudden displacement of water. They travel at high speed water as low broad waves and build to great heights as they approach shores. Origin including volcanic or volcano-tectonic earthquakes, explosions collapse or subsidence, landslides, lahars, pyroclastic flows or debris avalanches entering bodies of water, and atmospheric waves that couple with the sea.


Debris avalanche - fast downhill movement of soil and rock, speed: 70 km/hr (due to high water content and steep slopes) caused by slope failure on the cones of stratovolcanoes

Hydrothermal explosions - explosions from instantaneous flashing of steam upon contact with hot rocks

Secondary explosions are caused by the contact of water with hot pyroclastic flow deposits.

Subsidence is a ground deformation resulting from the downward adjustment of surface materials to the voids caused by volcanic activity.

This may result also from mine workings or geothermal water or oil extraction.

Precursors of an Impending Volcanic Eruption

The following are commonly observed signs that a volcano is about to erupt.  These precursors may vary from volcano to volcano.


1. Increase in the frequency of volcanic quakes with rumbling sounds; occurrence of volcanic tremors

2. Increased steaming activity; change in color of steam emission from white to gray due to entrained ash

3. Crater glow due to presence of magma at or near the crater

4. Ground swells (or inflation), ground tilt and ground fissuring due to magma intrusion

5. Localized landslides, rockfalls and landslides from the summit area not attributable to heavy rains

6. Noticeable increase in the extent of drying up of vegetation around the volcano's upper slopes

7. Increase in the temperature of hot springs, wells (e.g. Bulusan and Canlaon) and crater lake (e.g. Taal) near the volcano

8. Noticeable variation in the chemical content of springs, crater lakes within the vicinity of the volcano

9. Drying up of springs/wells around the volcano

10. Development of new thermal areas and/or reactivation of old ones;appearance of solfataras.




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